Thomas F. Nägler

Molybdenum isotope records as a potential new proxy for paleoceanography

New high-precision isotope ratios of dissolved Mo in seawater from different ocean basins and depths show a homogeneous isotope composition (‘mean ocean water 98 Mo/ 95 Mo’ (MOMO)), as expected from its long ocean residence time (800 kyr). This composition appears to have been constant for the past 60 Myr at a 1^3 Myr time resolution as indicated from thick sections of Fe^Mn crusts from the Atlantic and Pacific. These records yield a constant offset from MOMO (average of 33.1 and 32.9x). They are similar to our new data on recent oxic Mo sinks: pelagic sediments and six Fe^Mn crust surface layers range from 32.7 to 32.9x and 32.7 to 33.1x, respectively. Recent suboxic Mo sinks from open ocean basins display heavier and more variable isotope ratios (30.7 to 31.6x relative to MOMO). Crustal Mo sources were characterized by measuring two granites (and a mild acid leach of one granite), seven volcanic rocks and two clastic sediments. All show a narrow range of compositions (32.0 to 32.3x). These data indicate that isotope fractionation by chemical weathering and magmatic processes is insignificant on a global scale. They therefore represent good estimates of the composition of dissolved Mo input to the oceans and that of the average continental crust. Thus, the Mo input into the oceans appears to be distributed into lighter oxic sinks and heavier reducing sinks. This is consistent with steady-state conditions in the modern ocean. The constant isotope offset between oxic sediments and seawater suggests that the relative amounts of oxic and reducing Mo removal fluxes have not varied by more than 10% over the last 60 Myr. An equilibrium fractionation process is proposed assuming that Mo isotope fractionation occurs between (dominant) MoO 23 4 and (minor) Mo(OH)6 species in solution, of which the latter is preferentially scavenged. @ 2003 Elsevier Science B.V. All rights reserved.

Model for kinetic effects on calcium isotope fractionation (δ44Ca) in inorganic aragonite and cultured planktonic foraminifera

The calcium isotope ratios (δ44Ca = [(44Ca/40Ca)sample/(44Ca/40Ca)standard −1] · 1000) of Orbulina universa and of inorganically precipitated aragonite are positively correlated to temperature. The slopes of 0.019 and 0.015‰ °C−1, respectively, are a factor of 13 and 16 times smaller than the previously determined fractionation from a second foraminifera, Globigerinoides sacculifer, having a slope of about 0.24‰ °C−1. The observation that δ44Ca is positively correlated to temperature is opposite in sign to the oxygen isotopic fractionation (δ18O) in calcium carbonate (CaCO3). These observations are explained by a model which considers that Ca2+-ions forming ionic bonds are affected by kinetic fractionation only, whereas covalently bound atoms like oxygen are affected by kinetic and equilibrium fractionation. From thermodynamic consideration of kinetic isotope fractionation, it can be shown that the slope of the enrichment factor α(T) is mass-dependent. However, for O. universa and the inorganic precipitates, the calculated mass of about 520 ± 60 and 640 ± 70 amu (atomic mass units) is not compatible with the expected ion mass for 40Ca and 44Ca. To reconcile this discrepancy, we propose that Ca diffusion and δ44Ca isotope fractionation at liquid/solid transitions involves Ca2+-aquocomplexes (Ca[H2O]n2+ · mH2O) rather than pure Ca2+-ion diffusion. From our measurements we calculate that such a hypothesized Ca2+-aquocomplex correlates to a hydration number of up to 25 water molecules (490 amu). For O. universa we propose that their biologically mediated Ca isotope fractionation resembles fractionation during inorganic precipitation of CaCO3 in seawater. To explain the different Ca isotope fractionation in O. universa and in G. sacculifer, we suggest that the latter species actively dehydrates the Ca2+-aquocomplex before calcification takes place. The very different temperature response of Ca isotopes in the two species suggests that the use of δ44Ca as a temperature proxy will require careful study of species effects.

Evidence of hydration of the mantle wedge and its role in the exhumation of eclogites

Serpentinite samples from the Indus suture zone, representing a shallower part of a paleo-subduction zone, show lowgrade metamorphic recrystallization (chrysotile+magnetite ˛ magnesite ˛ talc). They are cumulates of melts formed in the uppermost mantle or the base of the Nidar intra-oceanic arc. Serpentinite samples associated with the Tso Morari eclogitic unit, representing the more deeply subducted portion of a paleo-subduction zone, exhibit high-grade metamorphic recrystallization (antigorite+magnetite ˛ forsterite ˛ talc) and the trace element chemistry of these samples suggests a strongly depleted mantle wedge origin. Nd concentrations and ONd values show that fluids responsible for hydration of the mantle wedge were derived from subducting clastic sediments overlying Tethyan oceanic crust. The exhumation of eclogites requires a mechanically weak zone at the interface between the subducting plate and the mantle wedge. We suggest that serpentinites associated with the Tso Morari eclogites acted as a lubricant for the exhumation of the eclogitic unit. Geophysical data suggest common occurrences of hydrated ultramafic rocks about 10 km thick along the interface between the mantle wedge and the subducting plate. We propose that such a low-viscosity zone played an important role for the exhumation of eclogitic rocks. fl 2001 Elsevier Science B.V. All rights reserved.

Sulfidity controls molybdenum isotope fractionation into euxinic sediments: Evidence from the modern Black Sea

Molybdenum (Mo) isotope fractionation has recently been introduced as a new proxy in oceanography and biogeochemistry. It is therefore fundamental to understand the processes controlling Mo partitioning into modern marine environments. This study identifies the availability of dissolved sulfide as the dominant control on overall Mo removal from the water column in euxinic systems. Mo isotopic composition of surface sediments from different localities of the Black Sea demonstrates complete fixation of Mo only below 400 m water depth, above a critical concentration of 11 μmol l −1 aqueous hydrogen sulfide in the bottom water. The Mo isotopic composition of these sediments reflects the homogeneous seawater isotopic composition of 2.3‰. In contrast, significant Mo isotope fractionation into less euxinic sediments is evident at shallower depths in the Black Sea, as well as in temporarily euxinic deeps of the Baltic Sea, consistent with the observed lower maximum sulfide concentrations in the respective water columns. Therefore, Mo isotope signatures in the modern Black Sea constrain the processes responsible for global Mo removal from the ocean by euxinic sediments. Furthermore, models of past ocean anoxia reconstruction have to consider that the seawater Mo isotopic composition is not per se archived in euxinic sediments.

Hydrogen sulphide release to surface waters at the Precambrian/Cambrian boundary.

Animal-like multicellular fossils appeared towards the end of the Precambrian, followed by a rapid increase in the abundance and diversity of fossils during the Early Cambrian period, an event also known as the ‘Cambrian explosion’. Changes in the environmental conditions at the Precambrian/Cambrian transition (about 542 Myr ago) have been suggested as a possible explanation for this event, but are still a matter of debate. Here we report molybdenum isotope signatures of black shales from two stratigraphically correlated sample sets with a depositional age of around 542 Myr. We find a transient molybdenum isotope signal immediately after the Precambrian/Cambrian transition. Using a box model of the oceanic molybdenum cycle, we find that intense upwelling of hydrogen sulphide-rich deep ocean water best explains the observed Early Cambrian molybdenum isotope signal. Our findings suggest that the Early Cambrian animal radiation may have been triggered by a major change in ocean circulation, terminating a long period during which the Proterozoic ocean was stratified, with sulphidic deep water.

Highly metalliferous carbonaceous shale and Early Cambrian seawater

We report evidence for the seawater origin of an extremely metal-enriched sulfide- and organic carbon–rich marker bed in a transgressive Early Cambrian black shale sequence along the passive margin of the Yangtze platform. The element concentration pattern in this marker bed suggests that it formed in a sediment-starved, stratified basin with a euxinic water column below an oxic surface layer. Biological activity was high in the surface layer, which was resupplied by communication with oxic oceans. The extremely low terrigenous input and the sulfate-reducing environment in the deeper part of the basin led to exceptionally high metal enrichments by factors of ∼107 with respect to modern seawater. The composition of the sulfidic rocks reflects the composition of the Early Cambrian oceans. The molybdenum isotope ratio suggests that during this time <35% of marine Mo was deposited in oxic sediments, and that suboxicanoxic marine environments were more widespread during the Early Cambrian than today.

The δ44Ca‐temperature calibration on fossil and cultured Globigerinoides sacculifer: New tool for reconstruction of past sea surface temperatures

We report direct δ44Ca-temperature calibration on cultured and fossil calcite foraminifera, showing that Ca isotopes are potentially a new proxy for past sea surface temperatures (SST). Samples have been analyzed using a 43Ca-48Ca double spike and thermal ionization mass spectrometry (TIMS). In order to avoid species-dependent isotope fractionation we focused our investigations on a single foraminifera species (Globigerinoides sacculifer), which is known to inhabit shallow euphotic waters in tropical and subtropical oceans. Ca isotope measurements were performed on cultured G. sacculifer that grew in seawater kept at temperatures of 19.5°, 26.5°, and 29.5°C. A δ44Ca change of 0.24 ± 0.02 per 1°C is defined by the weighted linear regression through reproduced δ44Ca data of the three temperatures (95% confidence level). Application of this new method to fossil G. sacculifer of an Equatorial East Atlantic sediment core (GeoB1112; 5°46.7′S, 10°45.0′W, 3125 m) indicates that the δ44Ca difference between marine isotope stage 1 (MIS-1) and MIS-2 is 0.71 ± 0.24. According to the current δ44Ca-temperature calibration this value corresponds to a temperature difference between MIS-1 and MIS-2 of ∼3.0 ± 1.0°C.

Weathering versus circulation-controlled changes in radiogenic isotope tracer composition of the Labrador Sea and North Atlantic Deep Water

Geological reconstructions and general circulation models suggest that the onset of both Northern Hemisphere glaciation, 2.7 Myr ago, and convection of Labrador Sea Water (LSW) were caused by the closure of the Panama Gateway ∼ 4.5 Myr ago. Time series data that have been obtained from studies of ferromanganese crusts from the northwestern Atlantic suggest that radiogenic isotopes of intermediate ocean residence time (Pb and Nd) can serve as suitable tracers to reconstruct these events. However, it has been unclear until now as to whether the changes that have been observed in isotope composition at this time are the result of increased thermohaline circulation or due to the effects of increased glacial weathering. In this paper we adopt a box model approach to demonstrate that the shifts in radiogenic isotope compositions are unlikely to be due to changes in convection in LSW but can be explained in terms of increases of erosion levels due to the glaciation of Greenland and Canada. Furthermore, we provide experimental evidence for the incongruent release of a labile fraction of strongly radiogenic Pb and nonradiogenic Nd from continental detritus eroding into the Labrador Sea. This can be attributed to the glacial weathering of old continents and accounts for the paradox that one of the areas of the world most deficient in radiogenic Pb should provide such a rich supply of radiogenic Pb to the oceans. An important general conclusion is that the compositions of radiogenic isotopes in seawater are not always a reflection of their continental sources. Perhaps more importantly, the transition from chemical weathering to mechanical erosion is likely to result in significant variations in radiogenic tracers in seawater.

Single mineral dating by the PbPb step-leaching method: Assessing the mechanisms

Stepwise PbPb leaching (PbSL) has been successfully used to date rock-forming silicates directly linked to metamorphic reactions defining a PT path. The two features of PbSL are an increase of precision and a control on accuracy: the former, by enhancing the 206Pb/204Pb and 207Pb/204Pb ratios, and the latter, by revealing heterochemical inclusions via the 208Pb/206Pb ratio and checking isotopic equilibrium with the host. The question of the need for inclusions as a prerequisite enabling PbSL dating was investigated on a centimenter-sized single crystal of museum-quality titanite. We obtained petrographic (optical microscope, SEM, electron microprobe, proton microprobe), chemical (ICP-MS), and isotopic (TIMS) data on pristine and increasingly leached splits of different grain sizes, as well as on leach solutions. The PbSL age of 1.00 Ga is identical to the concordant conventional UPb age. By use of isotopic and elemental correlation diagrams, we were able to resolve three isotopically distinct sources of Pb. Visible inclusions of K-feldspars as micro-crack fillings may contribute to the first 50% of common Pb (204Pb) release, and visible 5 μm zircons account, at least in part, for the residue. An additional effect noticed during the first leach steps is related to surface hydrolysis and consequent weakening of metal cation-oxygen bonds, which releases structurally bound common Pb substituting for Ca in the titanite lattice. The radiogenic Pb released during most leach steps has uniform 208Pb/206Pb ratios, suggesting that it is derived from a single mineral phase. Further, as different titanite grain sizes produce a different leach trajectory in Pb isotopic space, it can be concluded that this radiogenic Pb is actually derived from the titanite itself. Therefore, PbSL is indeed capable of discriminating common and radiogenic Pb from a single phase. During leaching, a well defined reaction front is seen to advance into the grains. Inside this front, the titanite is unaltered, and outside it, a silica-gel-like rim is formed, in which electron and proton microprobe data show retention of high field strength (HFS) elements. We propose that radiogenic Pb occurs in the tetravalent state (due to recoil stripping) so that it behaves as an HFS element and is preferentially retained in the gel rim during leaching, relative to common divalent Pb. The mechanism by which a spread in Pb isotope data is obtained during PbSL is, therefore, explained by two processes controlling the release of cations from a mineral grain into a leach solution: (1) an effective surface dependent hydrolysis of metal cations at the inward migrating reaction front whose influence decreases as leaching proceeds and (2) an increasingly dominant remobilization of HFS-elements and radiogenic Pb from the micro-environment of the leached gel-like layer which acts as a selective HFS cation absorber. In conclusion, Pb/Pb dating by stepwise leaching can be effective as a dating tool, even in minerals free from micro-inclusions.

Calcium isotope (δ44/40Ca) variations of Neogene planktonic foraminifera

Measurements of the calcium isotopic composition (δ44/40Ca) of planktonic foraminifera from the western equatorial Pacific and the Indian sector of the Southern Ocean show variations of about 0.6‰ over the past 24 Myr. The stacked δ44/40Ca record of Globigerinoides trilobus and Globigerina bulloides indicates a minimum in δ44/40Casw (seawater calcium) at 15 to 16 Ma and a subsequent general increase toward the present, interrupted by a second minimum at 3 to 5 Ma. Applying a coupled calcium/carbon cycle model, we find two scenarios that can explain a large portion of the observed δ44/40Casw variations. In both cases, variations in the Ca input flux to the ocean without proportional changes in the carbonate flux are invoked. The first scenario increases the riverine calcium input to the ocean without a proportional increase of the carbonate flux. The second scenario generates an additional calcium flux from the exchange of Ca by Mg during dolomitization. In both cases the calcium flux variations lead to drastic changes in the seawater Ca concentrations on million year timescales. Our δ44/40Casw record therefore indicates that the global calcium cycle may be much more dynamic than previously assumed.